The world’s No1 woman player, Hou Yifan, enters Saturday’s final round of the Grenke Classic at Baden-Baden in contention for a historic result.

After the first three of the seven rounds, played at Karlsruhe, the Chinese 23-year-old led the field on 2.5 points, crushing the world No4, Fabiano Caruana, then having much the better of a draw with Magnus Carlsen, the world champion. But in Friday’s penultimate round Hou lost to Armenia’s Levon Aronian, who leads on 5/6 ahead of Caruana and Carlsen.

Her results against Caruana and Carlsen were a landmark for women’s chess and a turnaround for Hou after her previous setbacks in 2017. At Tradewise Gibraltar she threw her final-round game in a bizarre and unjustified protest against the pairings; then, back on Chinese soil, she tamely lost her match 3-1 against the veteran Vassily Ivanchuk.

Karlsruhe was different. Caruana’s standard defence to 1 e4 is the Berlin Wall e5 2 Nf3 Nc6 3 Bb5 Nf6, but Hou had prepared in depth, took the initiative and increased her advantage when the 2016 US champion made nervous errors until she finally dominated the board.

Judit Polgar, the Hungarian legend and all-time No1 woman, never got near to beating the then world champion, Garry Kasparov, in classical chess but Hou had Carlsen on the ropes at Karlsruhe. The Norwegian, 26, made a rare choice of the Najdorf Sicilian in his attempt to score the full point, then carelessly allowed the standard idea Nf3-d2-c4 harassing Black’s d6 pawn.

Carlsen was reduced to passive defence until at the critical moment, despite a long think, Hou failed to find the strong plan of advancing her h pawn to h6 with mating threats. Short of time, she settled for a simplifying tactic and a level rook and pawn ending which was quickly drawn.

For the world champion matters are starting to become serious. Carlsen appeared at Karlsruhe wearing spectacles, explaining that he had always been near-sighted but had recently developed headaches for which glasses were the prescribed remedy.

The Norwegian drew all his first four games, despite having near-winning advantages against both the 20-year-old German Matthias Bluebaum and Armenia’s world No8, Levon Aronian. He dropped six Fide rating points, and his margin over the world No2 and US champion, Wesley So, shrunk to a mere 16 points, one of the narrowest leads since Carlsen became undisputed No1 in July 2011.

Carlsen’s rating peak in 2014 was 2882; three years later it is 2832 and drifting. He has often said that the No1 ranking matters more to him than even the global crown and a crisis could be approaching.

So is currently playing in Shamkir in Azerbaijan where his rivals are headed by Russia’s world No3, Vlad Kramnik, and the 2016 title challenger, Sergey Karjakin. England’s Michael Adams is also there, seeking a comeback after failing in his last event in China.

Wesley So’s hopes of closing in further on Carlsen and of extending his personal unbeaten run of 67 games received a setback in Friday’s opening round at Shamkir, where the US champion lost with the white pieces to Azerbaijan’s No1, Shak Mamedyarov, who always plays well in his home event.

Besides Baden-Baden and Shamkir, the other major event live online this weekend is the Reykjavik Open, where England’s Gawain Jones, fresh from his triumph at Dubai, is the No7 seed.

Hou stood slightly worse from the opening against Georg Meier due to her hanging central c5 and d5 pawns but after the German GM missed 19 Nc4! with a white edge and then 24 b4! (cxb4 25 Qxc8) with equality, she developed a powerful attack with the climax 29...Nxf2! opening up the white king to decisive threats from Black’s queen and rook.